The field experiment "VAL-STRESS3D", started in 2005 by INERIS, aims to characterize and study both in situ stress and continuous thermomechanical deformations within the "Rochers de Valabres" rock slope, a Pilot Site Laboratory (PSL) prone to rockfalls. Firstly, in situ stress measurements were conducted along a profile from the surface toward the interior of the slope. This was intended to quantify the stress field and its heterogeneity acting in such a geological structure. Secondly, a set of full 3D strain cells and temperature gauges was installed in the field around a large inclined superficial flagstone. This shallow temperature and strain instrumentation network was designed to measure the cyclic thermal "loading – unloading" response of the rock on a daily and seasonal basis.
Although they have potentially important effects on regional development in mountainous areas, rockfalls are a natural hazard for which both preparatory and triggering mechanisms are seldom understood. Lack of knowledge of the quantitative role of numerous factors remains a barrier to accurate risk assessment and reasonable identification of large-scale failure activation processes. Mitigation techniques remain expensive and, most of the time, do not definitely eliminate the risk. Setting up operational early-warning systems, when needed, quite often raises problems in terms of critical choices and monitoring strategies. These difficulties are related to the complexity of the studied system, i.e. the multiplicity of variables concerned and their interaction or coupling effects, making time failure unpredictable. In order to overcome these difficulties, a large scale fractured rock slope, namely the "Rochers de Valabres", located in the French Alps, and overhanging the Tinee valley, is being investigated as a Pilot Site Laboratory (PSL), by theCENARIS – the French National Monitoring Centre for Ground and Underground Risks, part of INERIS. The main objective is to run field experiments for research and technical development focusing on rock slope instabilities. Since 2002, this PSL has demonstrated its unique advantages for conducting long-term scientific work in the field, with successive projects feeding each other over time.
Lying at an altitude ranging from 700 to 900 m, the "Rochers deValabres" PSL is a specific zone of hard gneissic rocks.The top of the slope culminates at 2,250 m, having a slope angle of around 60°. The gneiss presents a marked foliation, directed N110°–140°E, strongly dipping eastward. More details may be found in Gunzburger et al., 2005. A glance at the bed of the river shows that this PSL has experienced and will likely continue to experience a rather large variety of rockfalls.The most recent history is marked by two important rockfalls, which occurred respectively in May 2000 (most likely around 2,000m3 of material) and October 2004 (about 30m3 of material). The main road snaking down the Tinee Valley has been subject to many renovation works. An old access road hugging the edge of the valley is used now as a service road to the PSL.
